Generally, drop detection devices are used to detect ink drops ejected by printhead nozzles. Based on the detection of ink drops, the status of a particular nozzle may be diagnosed. Typically, a printhead ejects ink drops in response to drive signals generated by print control circuitry in a printer. A printhead that ejects ink drops in response to drive signals may be referred to as a drop on demand printhead. Typically, there are two commonly used drop on demand technologies. These technologies are thermal (or bubble-jet) inkjet printing and piezo-electric (or impulse) inkjet printing. In thermal inkjet printing, the energy for ink drop ejection is generated by resistor elements, which are electrically heated. Such elements heat rapidly in response to electrical signals controlled by a microprocessor and creates a vapor bubble that expels ink through one or more nozzles associated with the resistor elements. In piezo-electric inkjet printing, ink drops are ejected in response to the vibrations of a piezo-electric crystal. The piezo-electric crystal responds to an electrical signal controlled by a microprocessor.
Nozzles through which ink drops are ejected may become clogged with paper fibers or other debris during normal operation. The nozzles may also become clogged with dry ink during prolonged idle periods. Generally, printhead service stations are used for wiping the printhead and applying suction to the printhead to clear out any blocked nozzles. The ink drop detectors may be used to determine whether a printhead actually requires cleaning. Additionally the detectors may be used to detect permanent failures of individual nozzles that may be caused, for example, by the failure of heating elements (in thermal ink jets) or by the failure in the piezo-electric crystals (in impulse printers). Other examples are related to detection of nozzles which have failed to eject drops because of de-priming (losing detection devices may also be used to calibrate the nozzle position relative to other parts of the printing machine.
Typically only high end printing systems have a drop detection system due to cost constraints. Consequently, growing complexity of printheads and harsh competition in printer costs and performance require new solutions for improvement in speed and printed image quality.